Linear magnetic shaft position sensor monitoring changes in the inductance in a coil

ABSTRACT

A shaft position detection device for detecting a certain position on a shaft comprising: a movable shaft including a detection portion to be detected, a characteristic of which is different from the other portions; a position detection coil to which the movable shaft is freely inserted; an inductance detecting means for detecting changing of inductance between a case of the movable shaft being in the position detection coil and a case of the movable shaft being out of the position detection coil; in which a position of the detection portion in response to the changing of the inductance. The difference of the magnetic characteristic is defined by a cross section area of the shaft, winding a magnetic tape, forming a magnetic layer.

BACKGROUND OF THE INVENTION

The present invention relates to a shaft position detection sensor whichdecreases erroneous detections due to grease coated on a shaft. Theshaft position detection sensor magnetically detects a position on theshaft which is constructed in such a manner that a permeability or across section area of a detection portion to be detected is differentfrom the other portions.

In the case that a position on a ball screw is detected in a movementmechanism using a ball screw and a ball nut, a position detection sensoras described later has been known. An optical sensor is provided on theball nut while a portion where a reflectance rate being different fromother portions is provided on a certain portion of the ball screw, sothat the strength of the reflection light is detected.

However, in such position sensor optically detecting the position, it isnecessary to place the detecting unit extremely close to the ball screw.Therefore, the optical detection unit may be dirty due to grease coatedon a surface of the ball screw. As described above, detection accuracymay be deteriorated.

SUMMARY OF THE INVENTION

In order to solve the above mentioned problems, the present inventionprovides a shaft position sensor which magnetically detects a positionon a shaft which is constructed in a manner that a permeability of adetection portion to be detected is different from the other portions ora shaft which is made of a magnetic material and is structured in amanner that a cross section area of a detection portion to be detectedis different from that of the other portions. The shaft position sensorcomprises a detection coil into which the shaft is inserted, and adetection unit which detects a difference between an inductance detectedwhen a detection portion to be detected is in the detection coil and aninductance when a detection portion is out of the detection coil.

According to the invention, the shaft is constructed in a manner that apermeability or a cross section area of a detection portion to bedetected is different from that of the other portions, so that theposition on the shaft is detected by detecting the inductance of thedetection coil. Therefore, the sensor is not influenced due to thegrease coated on the shaft. Accordingly, the detection accuracy can beimproved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing a basic structure of a shaft positiondetection sensor of the present invention;

FIGS. 2A to 2C show a detection portion formed on a shaft, FIG. 2A showsa cross section area being changed at a detection portion, FIG. 2B showsa magnetic tape wound on the shaft, and FIG. 2C shows a magnetic layerformed on the shaft;

FIG. 3 shows a cross section of a coil and a detection portion where theshaft position detection sensor of the invention is applied to detect aposition on a ball screw;

FIG. 4 is a perspective view of the coil housing of FIG. 3;

FIG. 5 is an enlarged cross section of a primary portion showing astorage condition of coil;

FIG. 6, comprised of FIGS. 6A and 6B is an explanation diagram for asurface effect at edge portion of the shaft;

FIG. 7 is a block diagram showing a structure of the detection portionof the present invention; and

FIG. 8 is a time chart showing an operation of detection unit as shownin FIG. 7.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A shaft position detection sensor of the present invention will bedescribed with reference to the drawings.

With reference to FIGS. 1 and 2, the detection principle of the shaftdetection sensor of the invention is explained.

FIG. 1 shows a basic structure of the shaft position sensor 1. A shafthas a detection portion 2a to be detected the magnetic characteristic ofwhich is different from the other portions. For example, the followingsarrangements are selectable. First, as shown in FIG. 2A, a cross sectionarea of the detection portion 2a is smaller than that of the otherportions so that step portions 2a and 2b are formed. Second, as shown inFIG. 2B, a magnetic tape 2c is wound at the detection portion 2a. Third,as shown in FIG. 2C, a magnetic layer 2d is formed at the detectionportion 2a by deposition or sputtering so that the magneticcharacteristic of the detection portion 2a is made different from theother portion. The detection portion 2a is not limited at one portion.The detection portion 2a may be provided at plural portions along theshaft 2.

Reference numeral 3 denotes a detection coil. The shaft 2 is insertedinto the detection coil 3. The shaft 2 is straightly moved relative tothe detection coil 3 indicated by arrow A as shown in FIG. 1. Aninductance difference is detected between when a detection portion 2a isin the detection coil 3 and a case of a detection portion 2a is out ofdetection coil 3. In FIG. 1, the detection coil 3 is fixed, and theshaft is movable. Needless to say, the relationship between thedetection coil 3 and the shaft 2 may be reversed, that is, the detectioncoil 3 is movable and the shaft 2 is fixed.

Reference numeral 4 is a resonance portion which uses the detection coil3 as an inductance element. The output of the resonance portion 4 isapplied to an alternating component detection portion 5 at a subsequentstage. The alternating component detection portion 5 detects analternating component from the signal applied from the resonance portion4. An output signal of the alternating component detection portion 5 isapplied to a signal period detection portion 6 at a subsequent stage.The difference between the inductance of the detection portion 2a andthat of the other portions is detected as a change in period of thesignal detected by the signal period detection portion.

An output of the signal period detection portion 6 is applied to abinarize portion 7 at subsequent stage in order to convert it into abinary signal which represents whether the detection portion 2a of theshaft 2 is detected.

As described above, according to the invention, because the shaftposition is magnetically detected, the decline of the detection accuracydue to the grease coated on the shaft 2 is prevented in comparison withthe optical detection.

FIGS. 3 to 8 show an embodiment of the present invention. The embodimentrelates to the position detection of the ball screw to detect a strokeend or a home position. A cross section area of the ball screw is variedas shown in FIG. 2A.

FIG. 3 shows a partial view of the ball screw and a coil portion.

In the drawing, reference numeral 8 is a ball screw, outer surface ofwhich a spiral lead groove 8a of about 1 mm depth is formed. A detectionportion 9 to be detected is formed on the ball screw 8. A cross sectionarea is smaller than that of the other portions so that step portions 10are formed. Namely, a shallow groove 11 is formed on the ball screw 8 inthe depth d (about 0.15 mm). This shallow grove 11 is designed not toaffect the engagement of the ball screw 8 and ball nut. The width W ofthe shallow groove is made greater than the thickness of the coilhousing.

A coil 12 is accommodated in a coil housing 13. As shown in FIG. 4, thecoil housing 13 is composed of a mounting plate 14 made of Ni--Fe systemmaterial and a cylindrical coil accommodating part 15 which is providedon the mounting plate 14. An insertion aperture 16 is formed at a centerof the coil accommodating part 15 allowing the ball screw 8 to penetratethereinto. The coil housing 13 is fixed to the ball nut or a fixingbracket in a manner that the mounting plate 14 is attached by screws orthe like.

The coil accommodating portion 15 is composed of an outer case 15ahaving L-shaped cross section and an inner bobbin 15b having U-shapedcross section as shown in FIG. 5.

The outer case 15a is made of Ni--Fe system material such as permalloyin order to reduce influence of external magnetic flux or temperatureand to concentrate the sensing magnetic flux.

The inner bobbin 15b made of synthetic resin is provided to protect thecoil 12 from the disperse of grease from the ball screw 8.

The coil 12 is wound around the inner bobbin 15b and is disposed in aspace defined between the outer case 15a and the inner bobbin 15b. Thenumber of windings of the coil 12 is about 50 to 100 times, and thediameter of coil is about 0.05 to 0.1 mm. As seen in FIG. 4, the ends ofthe coil 12 are lead out from a hole 17 which is formed on an outercircumferential surface of the coil accommodating part 15 at a positionclose to the mounting plate 14.

In this embodiment of the invention, the following can be employed as adetection principle.

(a) In case of energization frequency for coil 12 being low, theinductance is varied in proportion to the cross section area of the ballscrew 8. Namely, the difference between the cross section area of thedetection portion 9 and that of the other portions is detected as achange of the inductance.

(b) In case of energization frequency being high, it is well known as asurface effect that if an edge is formed on the surface of the shaft,then the magnetic flux is concentrated to the edge. Therefore, in caseof energization frequency for coil 12 being high, the magnetic flux isconcentrated to the edge of the step portion 10 so that the inductancewill be varied due to this edge.

If the energization frequency for coil 12 is in a range from 500K to 1 MHz, the magnetic flux tend to pass the surface of the shaft as shown inFIG. 6A. If an edge exist on the shaft, the magnetic flux isconcentrated on the edge as shown in FIG. 6B. Therefore, when thedetection portion 9 passes the coil 12, the inductance will raiseseveral percent.

One can choose from the above mentioned two ways in consideration of thedepth of the groove 11, S/N ratio of the signal, and stability of asensitivity. Either way may be used even if a spiral lead groove 8a isformed on the ball screw 8 because the cross section area of the ballscrew 8 is constant irrespective of the rotational angle. Therefore, itis possible to decrease the detection error of the inductance.

FIG. 7 shows a specific example of the detection circuit. FIG. 8 showswaveform diagrams of the corresponding part of FIG. 7.

Reference numeral 19 is a resonance circuit which uses the coil 12 as aninductance circuit. For example, a modified Wien-bridge oscillationcircuit is used. An output signal S19 is reversed to the detectionsignal S12 of the coil 12 in relationship of the leading and trailingedges. In the drawings, reference "T" represents a period and "ΔT"represents a variation of the period in response to changing of theinductance.

The output signal S19 of the resonance circuit 19 is sent to a buffer 20to be wave-shaped, and then sent to a counter 21 as a clock signal.

The counter divides the output signal 20 into 1/n (n≧2, n being integralnumber) so that detection accuracy of the inductance change is amplifiedto n times. (See period n×(T+ΔT)). Accordingly, the change of inductanceis made easy to detect.

An output signal S21 of the counter 21 is subsequently sent to amono-multi-vibrator 22 and a retriggerable mono-multi-vibrator 23 tomeasure a period of the signal.

The mono-multi-vibrator 22 is designed to have an adjustable referencetime constant generated by an external resistor and a capacitor so thatan output pulse whose pulse width is in response to the inductancechange is obtained with the signal width of the reference time constantresponsive to the trailing timing of the output signal S21 of thecounter 21. Accordingly, a threshold level for inductance ispredetermined so that the reference time constant is determined to bezero pulse width if the inductance is lower than the predeterminedthreshold level. In FIG. 8, the width ΔU of the signal S22 is varied inresponse to the change of the inductance. For example, the thresholdlevel is determined as an intermediate value between the inductance ofthe detection portion of the ball screw and that of the other portions.

The retriggerable mono-multi-vibrator 23 receives an output pulse fromthe mono-multi-vibrator 22 so that the reference time constant (see RC'in FIG. 8) is set by an external resistor and a capacitor in a mannerthat the vibrator 23 outputs "0" in case of the pulse width being zerowhile outputs "1" in case of the pulse width being not zero. (It shouldbe noted that it is negative logic.) Then, the binarized signal is sentto an open-collector transistor 24 to obtain a detection signal.

As described above, according to the invention, the detection portion ofwhich a permeability is different from that of the other portions in theshaft is detected by the inductance change in response to whether thedetection portion is located in the detection coil. Further, thedetection portion having a cross section area different from that of theother portions in the shaft made of magnetic material causes theinductance change in response to whether the detection portion islocated in the detection coil. Accordingly, the detection accuracy isprevented from lowering due to grease coated on the shaft.

Moreover, according to the invention, a magnetic tape may be wound onthe shaft or a magnetic layer may be formed to define the detectionportion. That is, it is possible to form a detection portion after theshaft has been produced.

Still further, according to the invention, one can choose to detect theinductance change in proportion to the cross section area of the shaftor the inductance change due to an edge between the detection portionand the other portions, according to the energization frequency appliedto the detection coil.

Still further, according to the invention, the alternating component ofthe oscillation signal of the resonance portion, which uses thedetection coil as an inductance element, is detected. Then, the periodof the alternating component is detected and is binarized so that theposition of the detection portion is detected in a relatively simplemanner.

Still further, the invention is applied to the ball screw of which across section area is constant even if a lead groove is formed on thesurface. Therefore, it is possible to decrease the unevenness of thedetection accuracy.

What is claimed is:
 1. A shaft position detection device comprising:amovable shaft including a detection portion to be detected, acharacteristic of which is different from other portions of said movableshaft; a position detection coil into which said movable shaft is freelyinserted; an inductance detecting means connected to the positiondetection coil for detecting a change in inductance between a case ofsaid detection portion being in said position detection coil and a caseof said detection portion being out of said position detection coil;wherein a position of said detection portion is detected in response tosaid change in inductance; wherein said inductance detecting meansincludes: an oscillation portion using said position detection coil asan inductance element, and generating an output signal having anoscillation frequency corresponding to the inductance of the inductanceelement; a period detection portion for detecting a period of the outputsignal of said oscillation portion; a comparison and detection portionfor comparing the period detected by said period detection portion witha predetermined threshold value, and for detecting a change in theinductance according to the output of the comparison.
 2. The shaftposition detection device as claimed in claim 1, wherein said comparisonand detection portion binarizes said output of the comparison andoutputs the binarized signal as a detection signal.
 3. The shaftposition detection device as claimed in claim 2, further comprising analternating component generation means for wave shaping the outputsignal of said oscillating portion to generate a periodic alternatingsignal;wherein said period detection portion detects the period from thealternating signal generated by said alternating component generationportion.
 4. The shaft position detection device as claimed in claim 3,wherein said period detection portion divides said periodic alternatingsignal into 1/n to detect the period.
 5. A shaft position detectiondevice comprising:a movable magnetic shaft including a detection portionto be detected, a characteristic of which is different from all otherportions; an edge formed by a change in the cross section area of saidmovable shaft at a boundary between said detection portion and the otherportions; a position detection coil into which said movable shaft isfreely inserted; an inductance detecting means connected to the positiondetection coil for detecting the change in inductance between a case ofsaid detection portion being in said position detection coil and a caseof said detection portion being out of said position detection coil;wherein a position of said detection portion is detected in response tothe changing of said inductance; wherein said inductance detecting meansincludes: an oscillation portion using said position detection coil asan inductance element, and generating an output signal having anoscillation frequency corresponding to the inductance of the inductanceelement; a period detection portion for detecting a period of the outputsignal of said oscillation portion; a comparison and detection portionfor comparing the period detected by said period detection portion witha predetermined threshold value, and for detecting a change in theinductance according to the output of the comparison.
 6. The shaftposition detection device as claimed in claim 5, further comprising analternating component generation means for wave shaping the outputsignal of said oscillating portion to generate a periodic alternatingsignal; wherein said period detection portion detects the period fromthe alternating signal generated by said alternating componentgeneration portion.
 7. The shaft position detection device as claimed inclaim 6, wherein said period detection portion divides said periodicalternating signal into 1/n to detect the period.